La-doped MIL-88B(Fe)–NH2: a mixed-metal–organic framework photocatalyst for highly efficient reduction of Cr(vi) in an aqueous solution

With the aim to resolve the problem of water pollution, we herein propose a new photocatalyst based on metal–organic frameworks (MOFs), called La-doped MIL-88B(Fe)–NH2 (MIL-88B((1 − x)Fe/xLa)-NH2), which was designed and employed for the photocatalytic reduction of Cr(vi) in aqueous solutions. MIL-88B((1−x)Fe/xLa)-NH2 materials with different x values were synthesized via a one-pot solvothermal method. Their characteristics were investigated using various techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), Brunauer–Emmett–Teller (BET) analysis, Fourier-transform infrared (FT-IR) spectroscopy and ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS). We found that compared to pristine MIL-88B(Fe)–NH2 with a photocatalytic efficiency of 67.08, MIL-88B((1 − x)Fe/xLa)-NH2 materials with x = 0.010, 0.025 and 0.050 exhibit excellent photocatalytic efficiencies reaching 88.21, 81.19 and 80.26%, respectively, after only 30 minutes of irradiation at a small catalyst dosage of 0.2 g L−1. These La-doped MIL-88B(Fe)–NH2 photocatalysts can work well under mild conditions (pH = 6). Furthermore, they are robust—can be recycled for at least four consecutive runs without any activity loss. This novel material is promising for the photocatalytic degradation of pollutants.


Introduction
The increase in toxic heavy metal ion contamination in aquatic environments has become a serious issue worldwide.Hexavalent chromium (Cr(VI)) is a typical contaminant that is widely used in various industrial elds such as electroplating, leather tanning, cooling tower blowdown, and rinse waters. 1 Waste Cr(VI) compounds are discharged easily into water bodies and cause serious harm to human health and organisms. 2Alternatively, Cr(III) is an essential trace metal involved in protein structure stabilization and glucose and lipid metabolism. 3herefore, reducing Cr(VI) to Cr(III) is considered an effective way for Cr(VI) removal from water.
A number of methods, including chemical, electrochemical and biological processes, are applied to aqueous Cr(VI) reduction. 4Therein, the reduction of Cr(VI) to Cr(III) via a photocatalysis process is a fruitful method.This photocatalytic technique is based on the electron/hole (e − /h + ) pairs generated in semiconductor materials under light illumination whose photon energy is greater than the semiconductor's bandgap energy. 57][8] However, its catalytic efficiency is limited by its large bandgap energy (3.2 eV) and the high recombination rate of photogenerated e − /h + pairs. 9etal-organic frameworks (MOFs) are a class of porous and crystalline materials composed of metal ions/ion clusters and organic ligands.Large specic surface area, structural tunability and reversible adsorption are the outstanding features of MOFs. 10 As a result, MOFs can be applied to a series of applications such as catalysis, 11,12 gas storage and separation, 13,14 cell imaging, 15 and sensing. 16In the catalysis area, in particular photocatalysis, MOFs have become dominant photocatalysts for treating water pollution because of their low e − /h + recombination probability due to ligand-to-metal charge transfer (LMCT). 17,18Fe-based MOFs (Fe-MOFs) are a family of potential materials in this eld owing to their relatively small bandgap in the range of 1.6-2.8eV, 11,12,[19][20][21][22] low toxicity and intrinsic stability. 23Moreover, Fe-MOFs contain unsaturated iron(III) ions with high catalytic activity, and this ensures their catalytic ability in advanced oxidation processes (AOPs), in particular Fenton-like processes. 23Among them, MIL-88B(Fe)-NH 2 (MIL: Materials of Institute Lavoisier) is a common Fe-MOF material whose structure is built up by trimers of iron(III) octahedra and 2-aminoterephthalate ligands. 24Compared to other MOFs, MIL-88B(Fe)-NH 2 exhibits high catalytic ability, 25 chemical stability, structural exibility, and abundant raw sources. 16Hence, it attracts remarkable attention in a wide range of applications such as heterogeneous catalysis, 26 adsorption, 27 sensing 16 and batteries. 28any strategies have been used in order to enhance the photocatalytic efficiency of MOFs as well as other semiconductors.0][31] Opposite to d-block metals, REEmetals have unique electronic properties because of their 4f electron congurations that are shielded from outermost subshell 5s and 5p, and REE-metals have distinct electronic and magnetic properties that are not signicantly altered by coordinating ligands.Furthermore, REEs in general and lanthanum ( 57 La) in particular are able to act as electron traps thanks to a plenty of empty orbitals in 4f and 5d subshells, thereby slowing down the e − /h + recombination rate and consequently improving the efficiency.The application of the Lanthanum-MOFs has been reported in various elds of catalysis, adsorption of toxic and heavy metal ions, and sensing.Further modi-cation of La-Fe MOFs can improve the surface area and catalytic capability of the materials.In this work, we aimed to synthesize and apply La-doped MIL-88B(Fe)-NH 2 for the photocatalytic removal of Cr(VI).Various methods including X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), Brunauer-Emmett-Teller (BET) analysis, Fourier-transform infrared (FT-IR) spectroscopy, and ultraviolet-visible diffuse reectance spectroscopy (UV-vis DRS) were employed to characterize the as-synthesized photocatalysts, and ultraviolet-visible (UV-vis) spectroscopy was used to determine the remaining Cr(VI) concentration in aqueous media.We found that the introduction of lanthanum(III) into the MIL-88B(Fe)-NH 2 structure enhances the efficiency of Cr(VI) removal.Besides, experiments with different lanthanum(III) contents were conducted to nd out the inuence of the mixed lanthanum(III) content on the efficiency of the Cr(VI) photoreduction.

Characterizations of materials
Fig. 1 shows the XRD spectra of MIL-88B((1 − x)Fe/xLa)-NH 2 (x = 0.010, 0.025, 0.050 and 0.10) materials (abbreviated in the graph: MIL(FeLa)) under the solvothermal condition of 150 °C and 12 hours.As reported in our previous work on MIL-88B(Fe)-NH 2 , 16 two characteristic peaks of the MIL-88B(Fe)-NH 2 phase appeared at 2q z 9.3 and 10.6°corresponding to the (002) and (101) lattices (CCDC 647646).Two of these diffraction peaks also appear for the MIL-88B((1 − x)Fe/xLa)-NH 2 materials, but they record a slight variation.In particular, the peak of the (002) lattice moves to a smaller angular position on the XRD pattern of all La 3+ ratios (2q z 9.2°).With the (101) lattice, the peak shis to the position 2q z 10.3°in the samples with x = 0.010 and 0.025, 2q z 10.8°in the sample with x = 0.050.More characteristic peaks are observed at 2q z 11.9°(sample with x = 0.10) and 2q z 20.6°(samples with x = 0.010 and 0.025) corresponding to the (102) and (202) lattice surfaces of MIL-88B(Fe)-NH 2 (CCDC 647646).These shis as well as the appearance of additional peaks on the XRD spectra of the Ladoped MIL-88B(Fe)-NH 2 material are the result of the presence of La 3+ in the MIL-88B(Fe)-NH 2 structure. 32Importantly, no impure phases exist in the MIL-88B((1 − x)Fe/xLa)-NH 2 (x = 0.010, 0.025 and 0.050) spectra, demonstrating the single-phase material.By contrast, the obtained MIL-88B((1 − x)Fe/xLa)-NH 2 (x = 0.10) material is impure, as proven by the appearance of strange peaks in the range 2q z 12.5-15.0°.It could be the result of ligand competition between La 3+ and Fe 3+ , or/and the formation of other lanthanum compounds.The ligand competition between La 3+ and Fe 3+ could lead to the formation of separate La-based MOFs.Many previous studies about MOF materials based on rare earth elements revealed that their MOF structure is complex and consequently difficult to determine because they are mostly built by metal ion chains with a large coordination number (usually 9). 33Besides, some La(III) compounds could form during the reaction such as La(NO 3 ) 3 , 34 LaCl 3 , 35 LaClO and La(OH) 3 . 36These ndings revealed that MIL-88B((1 − x)Fe/xLa)-NH 2 was successfully synthesized at x values equal to 0.010, 0.025 and 0.050.
8][39] Furthermore, a relatively uniform distribution of the particles in all materials is observed.The size distribution and mean size were calculated using the ImageJ soware, and the outcomes record changes in the particle size in the obtained MOFs.The average widths of MIL-88B((1 − x)Fe/ xLa)-NH 2 at x = 0.010, 0.025 and 0.050 are 462.5, 529.5 and 842.5 nm, respectively; corresponding to the average length/ width ratio of 3.42; 2.76 and 2.11 (Table 1).It can be seen that the width size is proportional to the La 3+ content, whereas the trend of the length/width ratio is opposite.The large radius of the La 3+ ion compared to Fe 3+ and the structural swell may cause this change.4).Normally, the S BET values of Fe-MOF materials are lower than those of other MOF families 40,41 due to their closed micropore structure (Table 2).Micropores in the Fe-MOFs' structure are incompatible with N 2 in terms of size, thereby restricting N 2 adsorption. 45,46Besides, the surface area of Fe-MOFs is affected by different synthesis conditions and methods as well.
where a is the absorption coefficient, h is Planck's constant, n is the photon's frequency, A is a proportionality constant and E g is the bandgap energy.
As shown in Fig. 6b, the bandgap energy (E g ) increases from 1.99, 2.00, 2.23 and 2.41 eV corresponding to the x value increasing from 0 (MIL-88B(Fe)-NH 2 ) to 0.050.It can be seen that La 3+ inserted into the MIL-88B(Fe)-NH 2 structure expands the bandgap energy, and this energy increases proportionally to the La 3+ content.This widening can be explained by the Burstein-Moss effect. 48La 3+ tends to contribute more electrons than Fe 3+ because the large ionic radius of La 3+ reduces the electrostatic interaction between the outer electrons and the nucleus, leading to the Fermi level being lled with electrons.Therefore, the following excited electrons can only move to an energy state higher than the Fermi level, causing the bandgap expansion.Moreover, crystal defects can be a factor that makes the E g value shi, and a decrease in crystal defects results in the E g increase. 49The binding energy of La-O is stronger than that of Fe-O (E La-O = 798 kJ mol −1 > E Fe-O = 407 kJ mol −1 (ref.50)) which contributes to reducing the number of defects in the crystal lattice.

Photocatalytic study
The results of UV-vis spectra of the Cr-DPC complex solution at various reaction intervals in the range of 2-14 minutes and its time-absorbance line graph are indicated in Fig. 7a and b.The output reveals that the maximum absorbance reaches a wavelength of 550 nm (l max = 550 nm), 51 and the absorbance of the Cr-DPC solution remains stable aer 11 minutes.Therefore, the absorbance of the following Cr-DPC solutions is measured at l max = 550 nm aer 11 minutes of reaction.To determine the linear range between the Cr(VI) concentration and the absorbance, we built two calibration curves in the Cr(VI) concentration range of 1-25 ppm (Fig. 7c and d).Linearity is observed from 1 ppm to 20 ppm using equation A = 0.0196[Cr(VI)] + 0.0030 (R 2 = 0.9988).
Previous studies have reported that the pH environment has considerable effects on the Cr(VI) photoreduction ability. 11,52,53nder a basic condition, the existing form of Cr(OH) 3 precipitation of Cr(III) can cover active sites on the catalyst surface    −0,13 V). 55 Besides, the Cr(OH) 3 solid can mask active sites, leading to the limitation of the material's catalytic activity.In the acidic environment, the lower the pH, the higher the redox potential of Cr(VI)/Cr(III) (E Cr(VI)/Cr(III) ), which is benecial for the reduction of Cr(VI) to Cr(III).Nonetheless, the results show the achieved highest performance at pH = 6.Thus, the Cr(VI) conversion performance can be affected by a Fenton-like process.Fe 3+ ions in the structure can react with H 2 O 2 , which increases the number of Fe 2+ ions via the Fenton mechanism  rate, thereby enhancing the catalytic activity. 30,58,59As such, the La 3+ appearance in the MIL-88B-NH 2 structure could inhibit the e − /h + recombination process, and the Cr(VI) photocatalytic efficiency here could be decided by the e − /h + recombination rate.
From the above-mentioned ndings, MIL-88B((1 − x)Fe/xLa)-NH 2 (x = 0.010) is selected to continue further research on the photoreduction process of Cr(VI) to Cr(III) in water.
In(C 0 /C t = kt) (1.1) where t is the reaction time (min); C 0 and C correspond to initial and remaining concentrations of Cr(VI) (mg L −1 ); k 1 (min −1 ) and k 2 (L mg −1 min −1 ) correspond to rate constants of the pseudo-1st-and 2nd-order models.
The stability of the MIL-88B((1 − x)Fe/xLa)-NH 2 (x = 0.010) photocatalyst was tested by recovering and reusing 4 times.Obviously, the photodegradation of Cr(VI) remains relatively stable in the subsequent runs, as shown in Fig. 12, indicating the high stability of the material.It can be seen that MIL-88B((1 − x)Fe/xLa)-NH 2 (x = 0.010) shows good photocatalytic ability with high performance and high stability in comparison to previous works. 19,53,60IL-88B(Fe)-NH 2 was reported to be an effective photocatalyst in the reduction of aqueous Cr(VI) thanks to the LMCT mechanism. 62,63Based on this, the proposed mechanism of MIL-88B((1 − x)Fe/xLa)-NH 2 (x = 0.010) for the photoreduction of Cr(VI) to Cr(III) in water is described as follows (Fig. 13): under   1)).The photoexcited electrons in the ligand move to the ion metal cluster, and this path is promoted more by amino groups.These generated electrons mainly participate in the reduction of Cr(VI) to Cr(III) (reaction ( 2)).The probability of e − / h + recombination is minimized thanks to the e − trapping ability of H 2 O 2 (reaction (3)), and the h + trapping ability of OH − ions (generated from reaction (3) 64 ) and H 2 O molecules (reactions ( 4) and ( 5)).Furthermore, Fe 2+ ions produced from the Fenton reaction and cO 2 − radicals produced from oxygen reduction by photoelectrons also contribute to Cr(VI) reduction (reactions (6-10)).

Chemicals and instrumentation
Iron(III) chloride hexahydrate (FeCl 3 $6H A Siemens D5005 diffractometer (Cu-K a radiation, l = 1.54056Å), a Hitachi S4800 scanning electron microscope, an ISIS 300 energy-dispersive X-ray spectrometer, a Gemini VII 2390 surface analyzer, a NICOLET iS50FT-IR spectrometer and a V-750 UV-visible spectrophotometer were used to perform XRD, SEM, EDS, BET, FT-IR and UV-vis DRS measurements, respectively.An Agilent 8453 UV-visible spectroscopy system was used to support for the determination of Cr(VI) concentrations.

Synthesis of La-doped MIL-88B(Fe)-NH 2
The fabrication process of La-doped MIL-88B(Fe)-NH 2 was referred from that of MIL-88B(Fe)-NH 2 , as reported in our previous work. 16In particular, MIL-88B((1 − x)Fe/xLa)-NH 2 materials were synthesized by an one-pot solvothermal method using a DMF solvent with a xed molar ratio of H 2 N-C 6 H 3 -1,4-(COOH) 2 (NH 2 -TPA) ligand to metal ions of 1.5, under reaction conditions of 150 °C and 12 hours (x: the molar ratio of La 3+ to the molar total of the metal ion, ).An appropriate amount of FeCl 3 $6H 2 O and La(NO 3 ) 3 $6H 2 O, and 0.6268 g NH 2 -TPA were dissolved into 50 mL DMF so that x reaches values of 0.010, 0.025, 0.050 and 0.10.The obtained solutions were sealed in autoclaves and then heated at 150 °C within 12 hours.Aerward, solid products were washed with DMF, methanol and distilled water and subsequently dried in a vacuum dryer.The fabricated MIL-88B((1 − x)Fe/xLa)-NH 2 materials are in a brown-colored powder form.

Photocatalytic experiments
The photocatalytic performance of La-doped MIL-88B(Fe)-NH 2 materials was studied through the photocatalytic reduction of Cr(VI) using a Hg lamp (250 W) as an ultraviolet light source.
The photoreduction efficiency of MIL-88B((1 − x)Fe/xLa)-NH 2 (x = 0.010) towards aqueous Cr(VI) solutions with different pH values was evaluated to study the inuence of pH environment on the material's photocatalysis activity.The investigated pH values were in the range of 2-8, adjusted by HCl and NaOH.Particularly, the photocatalyst was dispersed in the Cr(VI)-containing solution (initial Cr(VI) concentration: 20 ppm; photocatalyst dosage: 0.2 g L −1 ) in a glass beaker in the darkness until an adsorption-desorption equilibrium was reached.Next, 3% H 2 O 2 solution was added into the reaction system (1 mL L −1 ), and illuminated at the same time.Subsequently, the mixture was collected at determined intervals, and the catalyst was separated by centrifugation.The Cr(VI) concentration was then determined by a diphenylcarbazide method.In this method, Cr(VI) ions react with 1,5-diphenylcarbazide (DPC) ligands to form a purple-coloured complex (Cr-DPC complex) under the acidic condition. 51,65er nding out the optimal pH condition, similar experiments were performed to assess the Cr(VI) photoreduction ability of MIL-88B(Fe)-NH 2 and MIL-88B((1 − x)Fe/xLa)-NH 2 (x = 0.010, 0.025, and 0.050) materials, impacts of H 2 O 2 appearance on the Cr(VI) photoreduction, photocatalytic kinetics and reusability of the photocatalyst.

Conclusions
In summary, a series of MIL-88B((1 − x)Fe/xLa)-NH 2 materials have been synthesized via a one-pot solvothermal approach and characterized by various measurement techniques including XRD, SEM, SEM-EDS, BET analysis, FT-IR spectroscopy and UVvis DRS.The results indicate that MIL-88B((1 − x)Fe/xLa)-NH 2 materials were fabricated successfully at x = 0.010, 0.025 and 0.050.MIL-88B((1 − x)Fe/xLa)-NH 2 (x = 0.010, 0.025 and 0.050) materials were then used as photocatalysts for the aqueous Cr(VI) reduction.Compared to pristine MIL-88B(Fe)-NH 2 , Ladoped MIL-88B(Fe)-NH 2 materials display a better photocatalytic efficiency, and the best is achieved on MIL-88B((1 − x) Fe/xLa)-NH 2 (x = 0.010).In addition, the impact of the pH environment on the reduction performance of Cr(VI), the photocatalytic kinetics and reusability of this catalyst were studied.The output shows that the kinetics of photocatalytic reaction follows the pseudo-1st-order model, and the material exhibits high efficiency under the weak acidic condition and high stability aer 4 running cycles.The harvested knowledge in this work is expected to contribute to the development of mixed-MOFs in the catalysis area for wastewater treatment.

Fig. 5
shows infrared spectra of the NH 2 -TPA ligand, MIL-88B(Fe)-NH 2 and MIL-88B((1 − x)Fe/xLa)-NH 2 (x = 0.010) materials.Two peaks at 3462 and 3334 cm −1 are attributed to the asymmetric and symmetric stretching vibrations of N-H bonds, respectively.Similarly, two peaks appear at 1567 and 1367 cm −1 due to the asymmetric and symmetric C-O stretching oscillation.A peak at 1682 cm −1 represents the presence of the C]O group.The peaks at 1252 cm −1 and 766 cm −1 correspond to C sp 2 -N and C sp 2 -H bending vibrations.All of these summits are observed in the infrared graph of the TPA-NH 2 ligand and the as-synthesized MOFs.Additionally, in the spectrum of MIL-88B(Fe)-NH 2 and MIL-88B((1 − x)Fe/xLa)-NH 2 (x = 0.010), there appear other peaks which characterize new binding vibrations.Particularly, the peak appears at 3327 cm −1 due to the presence of O-H vibration that belongs to H 2 O molecules adsorbed in the MOF material.The characteristic vibrations of Fe-O and La-O bonds are observed at 507 cm −1 , evidence of the binding formation among Fe 3+ , La 3+ and the COO-groups in the ligand.As such, the FT-IR results contribute to conrming the bond formation of the metal centers with the ligand as well as the structural stability when induced by the La 3+ ion.Fig. 6a shows the UV-vis DRS results of MIL-88B(Fe)-NH 2 and MIL-88B((1 − x)Fe/xLa)-NH 2 (x = 0.010, 0.025 and 0.050) materials.Compared to the original MIL-88B(Fe)-NH 2 material, the wavelength at which the maximum absorption of MIL-88B((1 − x)Fe/xLa)-NH 2 materials takes place does not change signicantly, but there is difference in absorption intensity.MIL-88B(Fe)-NH 2 has the maximum wavelength (l max ) at 390 nm with an absorption edge extending to the visible light region while the absorbance in the spectra of the MIL-88B((1 − x)Fe/xLa)-NH 2 (x = 0.010, 0.025 and 0.050) drops slightly.The bandgap energies of MIL-88B(Fe)-NH 2 and MIL-88B((1 − x)Fe/xLa)-NH 2 (x = 0.010, 0.025 and 0.050) photocatalysts were determined using the Kubelka-Munk equation and the Tauc plot 47 as follows:

Fig. 8
Fig. 7 (a) Adsorption spectra and (b) time-absorbance line graph of the Cr-DPC complex.(c) Adsorption spectra of Cr-DPC complex solutions with different initial Cr(VI) concentrations and (d) linear relationship between the absorbance at 550 nm and Cr(VI) concentration.